Cisco Nexus 7000 Series Configuration Manual page 130

Configuring the Root Bridge ID
The spanning-tree vlan vlan_ID primary root command fails if the value required to be the root bridge
Note
is less than 4096. If the software cannot lower the bridge priority any lower, the device returns the following
message:
Error: Failed to set root bridge for VLAN 1
It may be possible to make the bridge root by setting the priority
for some (or all) of these instances to zero.
Caution The root bridge for each instance of STP should be a backbone or distribution device. Do not configure
an access device as the STP primary root.
Enter the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of bridge
hops between any two end stations in the Layer 2 network). When you specify the network diameter, the
software automatically selects an optimal hello time, forward delay time, and maximum age time for a network
of that diameter, which can significantly reduce the STP convergence time. You can enter the hello-time
keyword to override the automatically calculated hello time.
With the device configured as the root bridge, do not manually configure the hello time, forward-delay
Note
time, and maximum-age time using the spanning-tree mst hello-time, spanning-tree mst forward-time,
and spanning-tree mst max-age global configuration commands.
Before You Begin
Ensure that you are in the correct VDC (or enter the switchto vdc command).
SUMMARY STEPS
1. config t
2. spanning-tree vlan vlan-range root primary [diameter dia [hello-time hello-time]]
3. exit
4. (Optional) show spanning-tree
5. (Optional) copy running-config startup-config
DETAILED STEPS
Command or Action
Step 1
config t
Example:
switch# config t
switch(config)#
Step 2
spanning-tree vlan vlan-range root primary [diameter
dia [hello-time hello-time]]
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
114
Configuring Rapid PVST+ Using Cisco NX-OS
Purpose
Enters global configuration mode.
Configures a device as the primary root bridge. The
vlan-range value can be 2 through 4094 (except for reserved
VLAN values.) The dia default is 7. The hello-time can be
from 1 to 10 seconds, and the default value is 2 seconds.